Effect of Removal of the Root Tip on the Development of Enhanced Rb+ Absorption by Corn Roots

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Abstract
Samples of primary root tissue of corn (Zea mays L.) were aged either in CaSO(4) solution or in humid air, after which they were immersed for 10 minutes in a solution containing 0.1 mm(86)RbCl. Aging in solution, but not in humid air, enhanced the subsequent rate of Rb(+) absorption. Excision of roots before aging was followed by greater enhancement than when exicision followed aging. The time course of aging of 1-cm segments from different portions of the root showed decreasing response with increasing distance from the root cap. The aging response of apical segments (5-15 mm from the root cap) could be detected within 10 minutes and usually reached a maximum within 2 hours. Rb(+) absorption by apical segments (5-15 mm) aged without the tip (0-5 mm) was more than double that by apical segments whose tips were left attached until the end of the aging period. When apical segments without the tip were aged for 2 hours in the CaSO(4) solution in which seedlings had previously been grown for 24 hours, the rate of absorption was only 63% of samples aged in fresh solution. When apical segments were aged for 2 hours in fresh solution containing excised tips floating free in the solution, the rate of Rb(+) absorption was 20% less than in samples aged in solution containing no excised tips. The data presented in this study are interpreted to indicate that a water-soluble metabolite, originating in the root tip and translocated basipetally, inhibits Rb accumulation.